Passive silencers are conventionally used in many industrial applications to reduce sound propagating through pipes or ducts. Passive silencers can have baffles, acoustic expansion chambers, and/or acoustically adsorptive material, and typically work well at high to middle acoustic frequencies, but perform less effectively when frequencies drop below 500 Hertz. Baffles improve the performance of passive silencers, but severely restrict air or gas flow through the silencer. Flow restrictions are undesirable in many industrial applications because the flow restrictions often cause inefficiencies elsewhere in the industrial process.
In contrast to passive silencers, active sound control systems perform best on low frequency sound waves that have relatively long wavelengths. Also, in general, active systems do not restrict flow significantly. Active sound control systems cancel input acoustic waves propagating down a pipe by introducing a canceling acoustic wave to destructively interfere with the input acoustic waves so that the amplitude of output acoustic waves exiting from the pipe are reduced. It is typical to sense input acoustic waves with one or more input transducers (e.g. input microphones) and sense output acoustic waves with one or more error transducers (e.g. error microphones). The input transducers supply input signals to an electronic controller, and the error transducers supply error signals to the electronic controller. The electronic controller is programmed to model the acoustics in the pipe and filter the signals from the transducers (e.g. microphones) to generate a correction signal. The correction signal is supplied to an actuator (e.g. a canceling loudspeaker); and, in response to the correction signal, the actuator introduces the canceling acoustic wave to destructively interfere the input acoustic waves propagating down the pipe.
Since active sound control systems perform better at low frequencies and passive silencers perform better at middle to high frequencies, it is desirable to combine active and passive silencing components in a single unit to improve performance over a broad band of acoustic frequencies. The silencing requirements at various industrial applications can be extremely inconsistent, however, as can be the amount of allowable flow restriction. It is, therefore, desirable that such a combined active and passive silencer be easy to retrofit to match the needs of the industrial application in which it is used. It is further desirable that such a combined active and passive silencers be tightly packaged, and easy to install, disassemble and service.